The long term goal of this research plan is to understand the process of carcinogenesis. The plan focuses on a family of genes which are targets for cancer causing mutations referred to as the proto-oncogenes. Using thymic lymphomas induced in rats by Moloney murine leukemia virus (MoMLV), new oncogenes will be identified and characterized and the mechanism by which normal proto-oncogenes can be converted into diesease-causing oncogenes studied. MoMLV induces lymphomas by inserting its genome in or adjacent to proto-oncogenes, thereby mutating them into oncogenes. 15 precent of MoMLV-induced lymphomas have proviruses integrated next to a known proto-oncogene, c-myc. Clones of normal and virally-altered c-myc genes will be fused to the reporter gene chloramphenicol acetyltransferase (CAT), and the activities of a series of myc-CAT fusions containing variable amounts of viral and myc sequences will be measured to determine what viral and myc sequences are required for the alteration of c-myc expression induced by the virus. Nuclear runoff assays will be used to determine if the viral increase in c- myc expression is the result of a reversal of termination of the c- myc transcription. In addition to c-myc, other sites in the rat genome are found to be frequent integration regions. We have identified one such, designated Dsi-1, and will search for a transcript from this region. If a transcript can be identified, the transcribed region will be cloned, sequenced, and tested for oncogenicity as described below. Additional proviral integration sites will be cloned to search for additional frequent integration regions, which will be characterized as described for Dsi-1. An assay for the oncogenic activity of frequent integration regions will be developed which measures acceleration of lymphoma development in transgenic mice carrying virally altered frequent integration regions.